The classic sterling silver as we know it since medieval times comprises 92.5% silver and 7.5% copper by weight. It is known to be an easily tarnishable alloy. Its hardness in a soft annealed condition is known to be about 60 Vickers, and can be reversibly increased by age hardening typically up to about 110 Vickers. It is known that classic sterling silver age hardens due to a silver-copper miscibility gap. It is also known that silver-copper alloys show practically no age hardening when the concentration of copper is below about 5% by weight. It is not unusual for the modern sterling silver alloys to contain certain other base metals besides copper including tin, zinc, and indium. There are numerous sterling silver alloys that are commercially available and are described in literature. Some of these alloys are designed to improve casting characteristics such as form-filling and fluidity. Some of these alloys claim such features as higher as cast hardness, ability to be hardened by heat treatment (reversible hardenability) and high tarnish resistance.
U.S. Pat. Nos. 4,810,308 and 4,869,757 teach alloys with the small additions of tin and lithium that increase the aged hardness of sterling silver up to about 156 Vickers. The tarnish behavior of such alloys, however, is similar to that of classic sterling silver.
The resistance to tarnish of sterling silver alloys can be improved by lowering the copper content and adding other elements as shown in the examples below. The annealed hardness of these alloys lies within the range between 60-80 Vickers. Some of these alloys may be age hardened up to 135 Vickers.
Other relevant U.S. patents include:                U.S. Pat. No. 4,973,446 teaches low copper alloys that have an improved tarnish resistance. These alloys are soft and can not be age hardened.        U.S. Pat. Nos. 5,037,708 and 8,136,370 describe low tarnish silver alloys that contain from 4% to 15% by weight palladium. These alloys also contain from 0.5% to 1.75% by weight indium and/or zinc. Although, these patents do not teach such alloys with age hardening characteristics, they are most likely age hardenable due to Pd—Cu order-disorder transformation. A fairly high content of palladium significantly increases the cost of these alloys.        U.S. Pat. No. 5,039,479 describes palladium-free low tarnish alloys. These alloys typically do not show an appreciable age hardening.        U.S. Pat. No. 5,171,643 teaches electrical contact silver material that contains from 0.1% to 1.0% palladium by weight. Additions of palladium in these alloys are small, and do not contribute to age hardening.        U.S. Pat. No. 5,558,833 teaches silver-indium based alloys that are palladium-free and soft.        U.S. Pat. No. 5,817,195 describes high zinc, and low copper and nickel silver alloy compositions where nickel is within the range 0.25%-0.5% by weight. Such levels of nickel may cause allergenic skin reactions.        U.S. Pat. No. 5,882,441 palladium-free low tarnish alloy that is soft due to low copper content.        U.S. Pat. No. 6,406,664 describes palladium-free alloys. The resistance to tarnish in these alloys is achieved by additions of germanium. The hardness of these alloys is similar to that of the classic sterling silver.        U.S. Pat. No. 6,726,877 teaches another germanium-containing alloy that is palladium-free.        U.S. Pat. No. 6,841,012 describes anti-tarnish silver alloy with the additions of numerous elements except palladium.        U.S. Pat. Nos. 6,860,949 and 7,118,707 teach tarnish resistant platinum containing silver alloys. The hardness of such alloys is expected to be similar to that of the classic sterling silver. These alloys contain no palladium.        U.S. Pat. Nos. 7,128,871 and 7,128,792 teach another palladium-free silver alloys with low copper content. These alloys are soft and may not show reversible age hardening.        U.S. Pat. No. 7,198,683 describes tarnish resistant and age hardenable alloy. It contains no palladium.        